The as-cast structures of conventional spheroidal graphite cast iron pipes (particularly those produced by centrifugal casting) belong either to spheroidal graphite+pearlite+ferrite (a-type) or to spheroidal graphite +cementite+pearlite+ferrite (b-type), and as such, they are so brittle that practical strength cannot be obtained unless they are annealed.
This is because after a spheroidal graphite cast iron pipe is cast, in the case of centrifugal casting, it is allowed to cool to about 700.degree.-500.degree.C within the mold, (which is a cylindrical metal flask) when heat is absorbed by the mold through the sand lining formed on the inner surface of the mold, accelerating the cooling of the cast pipe to prevent the Ar.sub.1 transformation, with the result that pearlite remains in the structure. Even in the case of casting such pipe by a gravity casting method using sand molds, the same may be said if it is a thin-walled pipe.
The means heretofore employed as remedies therefore include decomposing pearlite by heat treatment for ferritization; increasing the Si content to 3.0% or above; and, in the case of centrifugal casting, increasing the thickness of the sand lining material in the mold (metal flask). However, the ferritization of pearlite by heat treatment requires a large amount of heat energy such as from oil, gas or the like and it also requires labor and facilities for heat treatment. Moreover, it raises problems including the pipe being caused to deform by heat treatment as well as reduction of tensile strength. On the other hand, the means resorting to the adjustment of Si content, though achieving ferritization in the as-cast condition, has the disadvantage of the pipe being brittle due to the high Si content, while the countermeasure resorting to increasing the thickness of the sand lining material to decrease the rate of cooling, through achieving ferritization in the as-cast condition, raises problems including the productivity being greatly decreased in view of the time involved in forming molds and the rate of cooling after casting.
A mold (in the form of a cylindrical metal flask) used in centrifugal casting is internally provided with a sand lining. Since this lining is generally as thin as 2mm or thereabouts formed as by the sand resin process, the heat in the cast pipe is quickly absorbed by said mold through the lining, thereby increasing the cooling rate of the cast pipe. If, therefore, the thickness of such lining is increased to about 25 - 40mm, it is possible to decrease the cooling rate. In that case, however, it is not possible to make use of simple means such as said sand resin process, and it becomes necessary to resort to a special molding procedure which requires a special molding installation and much labor and time. For example, it would involve placing said mold (which is generally as long as 6m) upright, setting a metal core concentrically within the mold to define an annular space of predetermined thickness between said mold and said metal core and pouring molding sand into the annular space while stamping the same.